1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems use a network of base stations to provide wireless connectivity to one or more mobile units. Each base station provides wireless connectivity within a geographical region that is referred to as a cell and/or a sector. Conventional base stations can transmit signals using a predetermined amount of available transmission power, which in some cases is approximately 35 W for a base station that serves a macrocell. The range of the macrocell is determined by numerous factors including the available transmission power, angular distribution of the available power, obstructions within the macrocell, environmental conditions, and the like. For example, the range of a macrocell can vary from as little as 300 m in a densely populated urban environment to as much as 10 km in a sparsely populated rural environment. The coverage area can also vary in time if any of these parameters changes.
Other types of access points or access networks can also be used to provide wireless connectivity to mobile units. For example, lower power access points (which may also be referred to as base stations, base station routers, home e-node-Bs, and the like) may be deployed in a business campus, a public area such as the train station or a food court, a residence or building to provide wireless connectivity to the occupants of the residence or the building. Base stations or access points deployed in a residence are typically referred to as home base station routers, home eNBs, femtocells, microcells, picocells, and the like because they are intended to provide wireless connectivity to a much smaller area (e.g., a microcell, femtocell, or picocell) that encompasses a residence. Low range access devices such as femtocells have a much smaller power output than conventional base stations that are used to provide coverage to macrocells. For example, a typical femtocell or microcell has a transmission power on the order of 10 mW. Consequently, the range of a typical femtocell is much smaller than the range of a macrocell. For example, a typical range of a femtocell is less than or on the order of about 100 m. Clusters of femtocells or microcells may also be deployed to provide coverage to larger areas and/or to more users.
Heterogeneous networks include a mixture of different types of devices for providing wireless connectivity to different sized cells. For example, femtocells are expected to be deployed in conjunction with a macro-cellular network in an overlay configuration. For another example, a macro-cellular network may be used to provide wireless connectivity to a neighborhood that includes numerous residences. A mobile unit traveling through the neighborhood or located in one of the residences can access the wireless communication system using the macro-cellular network. Individual femtocells can be deployed in one or more of the residences to provide overlay coverage within (or near) the residence. Clusters of femtocells can also be deployed in one or more of the buildings to provide overlay coverage within (or near) the building. In either case, there will be a one-to-many relationship between the macrocells and the femtocells within the coverage area. Heterogeneous networks can also include microcells, picocells, and relays that operate in different sized geographical areas. However, the devices deployed in heterogeneous networks are typically classified into two major types: (1) large cells that include macrocells and macrocell relays and (2) small cells that include microcells, pico cells, HeNBs, femtocells, and small relays.
As the user moves throughout the geographic areas served by the large cells and the overlaying smaller cells, the user equipment can be handed off between the large cells and/or the small cells. The basic condition for initiating a handover is that the signal strength from the candidate target base station or cell is stronger/better than the signal strength from the current serving base station or cell. However, simply handing off a mobile unit as soon as the target base station appears to have a stronger signal than the serving base station can lead to a number of problems. For example, the signal strengths near the boundaries between a serving cell and its neighbor cells are (almost by definition) nearly equal. The signal strength received by each mobile unit near a boundary is therefore approximately equal and relatively small deviations can cause the relative signal strengths to flip-flop. The strength of the signals received by a particular mobile unit may also vary rapidly due to movement of the mobile unit and/or environmental changes. Consequently, the mobile unit may be rapidly handed back and forth (a phenomenon known as ping-ponging) if the hand off is performed based only on the relative signal strength. Ping-ponging consumes valuable overhead unnecessarily, degrades the perceived call quality, and can lead to dropped calls.
Handovers can be made more robust by using a more sophisticated handoff condition. For example, conventional handovers are performed when the signal strength from the candidate cell is better than the signal strengths from the current serving cell by a certain amount determined by a hysteresis value and offset values. Each cell uses a single value of the hysteresis, e.g., 2 dB. Each cell also maintains different values for the offsets that are applied to handoffs between the cell and its neighbor cells. For example, the offset value for handoffs between a serving cell and a first neighbor cell may be 1 dB and the offset value for handoffs between the serving cell and a second neighbor cell may be 2 dB. A time-to-trigger (TTT) is used to delay the hand off until the “better” conditions on the target cell persist for at least the TTT duration. In 3G technologies, the hysteresis, offset values, and, TTT are set to one golden set that is applied to all cells.
However, the conventional handoff techniques used by active mobile units and cell reselection techniques used by idle mobile units do not distinguish between large cells and small cells. Consequently, heterogeneous networks may not be able to effectively direct and/or distribute traffic between large cells and potentially overlapping small cells. The inability to smoothly distribute users within the heterogeneous network may lead to load imbalances and other problems.
The operation of heterogeneous networks may be further complicated by the use of different standards and/or protocols for the different types of access points. Exemplary network services can be provided by different network elements using different carriers that operate according to different transmission protocols including High Rate Packet Data (HRPD), Long Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS), and the like. Each of these network services use carriers that can be defined in terms of a particular radio access technology (RAT) and the radio access technology that defines each different transmission protocol typically requires a unique radiofrequency configuration for transmission and reception of communications based on the radio access technology.